Oligonucleotides are important materials for research, diagnostic, therapeutic and other purposes. An ever-growing demand for improved oligonucleotides, oligonucleotide analogs and for methods for their preparation and use has arisen. For example, oligonucleotides are widely used in genomic research as probes, primers and a wide array of other research uses. One widely used technique that uses oligonucleotides primers is PCR (polymerase chain reaction) amplification.
Oligonucleotides are also useful in diagnostics since they can specifically hybridize to nucleic acids of interest in the etiology of disease. Oligonucleotides are currently in clinical trials as therapeutic moieties in the treatment of disease states. For example, workers in the field have now identified oligonucleotide compositions that are capable of modulating expression of genes implicated in viral, fungal and metabolic diseases. In short, oligonucleotides are important molecules having a large commercial impact in biotechnology and medicine. Improved methods for the synthesis of oligonucleotides are in demand, especially methods which have improvements in cost and convenience.
The current methods of choice for the preparation of phosphorothioate oligonucleotides employ solid-phase synthesis wherein an oligonucleotide is prepared on a polymer or other solid support. Solid-phase synthesis relies on sequential addition of nucleotides to one end of a growing oligonucleotide. Typically, a first nucleoside is attached to an appropriate support, e.g. glass, and nucleotide precursors, typically phosphoramidites, are added stepwise to elongate the growing oligonucleotide. The nucleotide phosphoramidites are conventionally reacted with the growing oligonucleotide using the principles of a "fluidized bed" for mixing of the reagents. The silica supports suitable for anchoring the oligonucleotide are very fragile and thus can not be exposed to aggressive mixing.
In these and other solid-phase procedures the oligonucleotide is synthesized as an elongating strand. However, the number of individual strands that can be anchored to a unit surface area of the support is limited. Also, the commercially available activated nucleotides that are presently used to add to a growing oligonucleotide are relatively expensive and must be used in stoichiometric excess. Also, the activating agents, e.g. tetrazole, are used in large excess.
The chemical literature discloses numerous processes for coupling nucleosides through phosphorous-containing covalent linkages to produce oligonucleotides of defined sequence. One of the most popular processes is the phosphoramidite technique (see, e.g., Beaucage, et al., Tetrahedron 1992, 48, 2223 and references cited therein), wherein a nucleoside or oligonucleotide having a free hydroxyl group is reacted with a protected cyanoethyl phosphoramidite monomer in the presence of a weak acid to form a phosphite-linked structure. Oxidation of the phosphite linkage followed by hydrolysis of the cyanoethyl group yields the desired phosphodiester or phosphorothioate linkage.
The phosphoramidite technique, however, is not without its disadvantages. For example, cyanoethyl phosphoramidite monomer is quite expensive. Although considerable quantities of monomer go unreacted in a typical phosphoramidite coupling, unreacted monomer can be recovered, if at all, only with great difficulty. Also, acrylonitrile, the by-product of deprotection of the cyanoethoxy group on the phosphate group is carcinogenic and in some cases acts as a Michael acceptor to form undesired side-products.
Other exemplary solid state synthetic schemes are set forth in U.S. Pat. Nos. RE. 34,069 Koster et al.; and 5,132,418 Carruthers et al.
While currently utilized solid phase syntheses are useful for preparing small quantities of oligonucleotide, they typically are not amenable to the preparation of large quantities of oligonucleotides necessary for biophysical studies, pre-clinical and clinical trials and commercial production. Moreover, such synthetic procedures are very expensive. Thus, although there is a great demand for oligonucleotides, especially phosphorothioate oligonucleotides, the art suggests no large scale techniques for their preparation. Accordingly, there remains a long-felt need for such methods and for intermediates useful in such methods.